cAMP-dependent protein kinase (cAPK) plays important roles in different celluar functions. The crystal structure of the catalytic subunit of cAPK is the first protein kinase structure determined and has served as a prototype for the entire protein kinase family. We have solved the crystal structure of a deletion regulatory subunit using the synchrotron radiation facility at SSRL. This structure has provided valuable insights into how cAPK interacts with the secondary messenger, cAMP. We are now producing crystals of a deletion cAPK holoenzyme complex formed by the catalytic subunit and the deletion regulatory subunit, delta (1-91)R. This holoenzyme crystal structure will first-time provide detailed information on how the catalytic and regulatory subunits interact thus, help us understand the mechanism of cAPK regulation. The deletion holo-complex crystal diffracted to 3.5 E at home source.